The invention relates to a particle-optical apparatus which includes
a particle source for producing a primary beam of electrically charged particles which travel along an optical axis of the apparatus,
a specimen holder for a specimen to be examined by means of the apparatus,
a focusing device for forming a focus of the primary beam in the vicinity of the specimen holder,
scanning means for scanning the specimen by means of the focused beam,
detection means for capturing electrically charged particles originating from the specimen.
An apparatus of this kind is known from U.S. Pat. No. 4,785,182.
Apparatus of the kind set forth are known as Scanning Electron Microscopes (SEM). In a SEM a region of a specimen to be examined is scanned by means of a primary beam of electrically charged particles, usually electrons, which travel along an optical axis of the apparatus. The acceleration voltage for the electron beam in the SEM is chosen in dependence on the nature of the specimen to be examined. This acceleration voltage should have a comparatively low value (of the order of magnitude of 1 kV) so as to minimize charging of the specimen by the primary electron beam. This could take place, for example during the study of electrically insulating layers in integrated electronic circuits or for given biological specimens. Moreover, for some examinations it is desirable that the electrons of the primary beam penetrate the specimen to a small depth only, resulting in a better contrast of the image to be formed. Other specimens, however, require a higher acceleration voltage, for example of the order of magnitude of 30 kV.
Irradiation of the specimen to be examined releases electrically charged particles (generally secondary electrons) which have an energy which is substantially lower, for example of the order of magnitude of from 5 to 50 eV. The energy and/or the energy distribution of these secondary electrons offers information as regards the nature and the composition of the specimen. Therefore, an SEM is attractively provided with a detector for secondary electrons. These electrons are released at the side of the specimen where the primary beam is incident, after which they travel back, against the direction of incidence of the primary electrons, approximately along the field lines of the focusing lens. Therefore, when a detector (for example, provided with an electrode carrying a positive voltage of 300 V) is arranged in the vicinity of the secondary electrons thus travelling back, the secondary electrons are captured by this electrode and the detector outputs an electric signal which is proportional to the electric current thus detected. The (secondary electron) image of the specimen is thus formed in known manner. With a view to the quality of the image, notably the speed at which the image is formed and the signal-to-noise ratio, the detected current is preferably as large as possible.
According to the cited United States patent the specimen to be examined is arranged in an atmosphere of a gas at a pressure of between 0.05 Torr (≈6.5 N/m2) and 20 Torr (≈2630 N/m2), so a pressure which is many times higher than the pressure at which conventional SEMs operate. The electric field produced by the voltage between the specimen and the electrode of the detector accelerates the secondary electrons emanating from the specimen to such a speed that they are capable of ionizing the atoms of the gas enveloping the specimen. During these ionizations, one or more electrons are released from the gas atoms, which electrons themselves are accelerated and can release further electrons by further ionizations again, etc. The gas surrounding the specimen thus acts as an amplifier for the secondary electron current, so that the current to be detected can in principle be larger than the current caused by the secondary electrons themselves.
Further advantages of a SEM operating with a gas atmosphere (to be referred to hereinafter as an xe2x80x9cEnvironmental SEMxe2x80x9d or ESEM) over the conventional SEM consist in that the ESEM enables the formation of electron optical images of humid or non-conductive specimens (for example, biological specimens, synthetic materials, ceramic materials or glass fibers) which are extremely difficult to image in the customary vacuum conditions in the conventional SEM. The ESEM enables the specimen to be maintained in its xe2x80x9cnaturalxe2x80x9d condition, without the specimen having to be subjected to the adverse effects of drying, freezing or vacuum coating operations which are normally required for the study by means of electron beams in high vacuum conditions.
Furthermore, because of the comparatively high permissible pressure in the specimen space of the ESEM, the gas ions formed neutralize any electric charging of a non-conductive specimen which could otherwise impede the formation of an image of high resolution. The ESEM also enables direct, real-time observation of phenomena such as transport of liquids, chemical reactions, solution, crystallization and other processes taking place at a comparatively high vapor pressure which is far beyond that permissible in the specimen space of a conventional SEM.
It is to be noted that, generally speaking, ESEMs can operate with an atmosphere in the specimen space whose pressure is also outside the range stated in the cited United States patent. It is notably possible to admit a lower pressure in the specimen space, for example a pressure of 0.01 Torr (≈1.3 N/m2).
It is a drawback of the device disclosed in the cited United States patent that a comparatively high voltage is required at the detector electrode so as to obtain a sufficient number of successive ionizations, and that hence the distance between the specimen and the detector electrode cannot become smaller than a comparatively large minimum distance.
It is an object of the invention to provide a scanning particle-optical apparatus in which the number of collisions between the ionizing electrons and the gas atoms becomes substantially higher than in the known particle-optical apparatus while using the same geometry of the specimen and the detector electrode.
To this end, the particle-optical apparatus according to the invention is characterized in that it includes means for producing an additional magnetic field in the space between the detection means and the specimen holder. In the context of the present invention the space between the detection means and the specimen holder is to be understood to mean the space which is traversed by electrically charged particles originating from the specimen (and possibly particles produced by said particles, for example electrons and ions produced by ionizations) before they are captured by a detector electrode.
It is to be noted that in the context of the present invention an additional magnetic field is to be understood to mean a magnetic field which is formed additionally to the magnetic field formed in the focusing device which serves to produce a focus of the primary beam, which field may possibly extend as far as the specimen as in the case of an immersion lens.
As is known, an electron moving in a magnetic field experiences a force which is directed perpendicularly to the direction of movement and also perpendicularly to the magnetic field. In the absence of a magnetic field, a secondary electron travelling from the specimen to the detector electrode will follow a practically straight path to the detector electrode, except for changes of direction due to collisions with gas atoms. In the presence of a magnetic field, therefore, such an electron is deflected away from its direction of movement to the detector electrode and, theoretically speaking, in the case of given field geometries it cannot even reach the detector electrode in the absence of loss of energy. As a result, this electron will travel a substantially longer distance, so that the probability of collisions with the gas atoms is substantially increased. Because of the ionizing collisions with the gas atoms, such an electron loses each time a given amount of energy during its travel, so that ultimately it can be captured by the detector electrode after all. During this considerably longer travel, therefore, this electron will have experienced a proportionally larger number of ionizing collisions, and hence have released a larger number of electrons. The same also holds for the electrons released by such collisions. Thus, a cascade of released electrons is formed, thus ensuring that the signal to be detected is much greater than in the absence of an additional magnetic field. This signal to be detected may take various forms, all of which constitute a representation of the current of electrons released from the specimen.
In an embodiment of the invention the detection of the signal to be detected takes place in that the signals originating from the specimen in response to the incidence of the primary beam are formed by electrically charged particles originating from the specimen. This current of electrically charged particles from the specimen may be the current of secondary electrons (i.e. the total number of electrons released from the specimen and the electrons produced by multiplication in the gas discharge). Alternatively, the current of electrically charged particles originating from the specimen is formed by a current of ions which arises in the gas discharge, moves to the specimen under the influence of the electric field and can be measured as a specimen current. A third possibility consists in that the current of electrically charged particles originating from the specimen is formed by a current of ions which arises in the gas discharge and moves, under the influence of, for example, an electric field produced by the detection means, to an electrode of the detection means and can be measured as a detector current. It is alternatively possible to combine two or more of the currents thus formed and to detect the signal then arising.
It is to be noted that in the context of the present invention the phrase xe2x80x9cthe space between the detection means and the specimen holderxe2x80x9d is to be understood to mean the space which is traversed by electrically charged particles originating from the specimen (and possibly particles caused by these particles, such as electrons and ions produced by ionization) before these particles are captured by a detector electrode.
In a further embodiment of the invention, the signal to be detected is detected in that the signals originating from the specimen in response to the incidence of the primary beam are formed by light signals which are generated by gas ionizations in the space between the detection means and the specimen holder. This effect is also amplified by the additional magnetic field, because it increases the path length of the electrons and hence the number of ionizations and also the amount of light generated thereby.
A preferred embodiment of the particle-optical apparatus according to the invention is provided with adjusting means for adjusting the strength of the additional magnetic field independently of the focusing device. This results in a high degree of flexibility of use of the apparatus, enabling the magnetic field to be adapted to a variety of imaging parameters which are subject to change, for example the distance between the specimen and the objective lens, the number of secondary electrons per primary electron, the acceleration voltage, the gas pressure in the specimen space, the number of ions required for discharging the specimen, etc.
The means for producing the additional magnetic field in a further preferred embodiment of the invention are arranged to produce a field shape which has practically no effect on the direction of the primary beam. Such a field shape can be readily realized by a person skilled in the art. Examples of such field shapes are a field shape which is rotationally symmetrical about the optical axis, a field shape having an n-tuple symmetry around the optical axis, as in the case of multipole fields extending perpendicularly to the axis, or a field shape with mirror symmetry in a plane through the axis. These field shapes offer the advantage that the primary beam is practically not disturbed by the additional magnetic field. Should such a field cause some disturbance nevertheless, it could be readily compensated by readjustment of the (rotationally symmetrical) field of the objective lens.
The focusing device in another embodiment of the invention, is formed by a magnetic slit lens and the means for producing the additional magnetic field are formed by a coil which is situated around the optical axis and on the outer side of the pole piece at the side of the lens which faces the specimen.
This embodiment offers the advantage that the field thus formed is automatically rotationally symmetrical about the optical axis. Any disturbances of the primary beam by the additional magnetic field can then be readily compensated by readjustment of the excitation of the likewise rotationally symmetrical slit lens.
The focusing device in a further embodiment of the invention is formed by a magnetic monopole lens with a pole piece which extends as a funnel around the optical axis and whose end of smaller diameter faces the specimen holder, the means for producing the additional magnetic field being formed by a coil which is arranged around the outer side of the funnel-shaped pole piece. This embodiment offers the advantage that the iron of the monopole lens is used to form the additional magnetic field, so that no or hardly any additional iron is required to form the field and the field thus formed is automatically rotationally symmetrically situated about the optical axis. Furthermore, the coil for generating this field then occupies only a comparatively small useful space in the specimen chamber.
The coil arranged around the outer side of the funnel-shaped pole piece in another embodiment of the invention is situated at the area of the end of smaller diameter thereof. In this manner the additional field is generated at the area where it is required most, i.e. in the vicinity of the space between the specimen and the lower side of the objective.
The means for producing the additional magnetic field in another embodiment yet of the invention are situated below the specimen holder, viewed from the electron beam. This embodiment is attractive notably when rotation symmetry of this field is necessary but the space in the vicinity of the monopole lens is required for other purposes, as in the case of large semiconductor wafers which must be tilted relative to the monopole lens for inspection.
The focusing device in another embodiment of the invention is provided with a magnetic circuit and the means for producing the additional magnetic field are formed by a magnetic circuit which is provided with a coil and is separate from the magnetic circuit of the focusing device. This embodiment can be advantageously used in situations in which the magnetic circuit may not be disturbed by other fields, or in which the manipulation of the additional magnetic field should be independent of the objective field for other reasons.
The magnetic circuit for the additional field in another embodiment of the invention has an E-shaped cross-section. Field concentrations can thus be readily generated in the vicinity of the central projection of the magnetic circuit so that, for example one or more electrodes for the detection of secondary electrons can be arranged at that area.
The detection means in a further embodiment of the invention include two grids which are arranged in the additional field and can be adjusted to a different electric potential. Secondary electrons can thus be pulled from the area of the specimen in the direction of the two electrodes by an electric field, the desired amplification of the detector current taken place at said electrodes because of the presence of the additional magnetic field. The additional magnetic field may again be attractively configured so as to be rotationally symmetrical about the optical axis.
The detection means in another embodiment yet of the invention are arranged to produce an electric multipole field around the optical axis which extends transversely of the optical axis in the same space as the additional magnetic field. This embodiment offers the advantage that a comparatively strong detector field can be realized while the primary electron beam is only slightly influenced. Moreover, the space angle at which the specimen is perceived from the electrode assembly producing the electric multipole field is thus made very large.
The detection means in a further embodiment of the invention are also arranged to produce a magnetic multipole field around the optical axis which extends transversely of the optical axis in the same space as the electric multipole field, said magnetic multipole field constituting the additional magnetic field. This embodiment avoids the necessity of separate means for generating the additional magnetic field.